Metal stanchions serve as the vertical structural posts of many shipboard safety railing systems. Stanchion-supported horizontal cables provide a ship""s crew with safety barriers along deck edges, platforms, and aircraft carrier elevators. One typical shipboard safety stanchion system consists of four primary parts, an approximately 38-inch long vertical hollow post with a 3.5-inch OD and 0.25-inch thick walls (the stanchion), a lower metal insert to facilitate mounting the post to the ship""s deck, an upper metal insert for interfacing the post with a post-cap, and a metal post-cap containing a through-hole to locate the horizontal cables that form the actual safety barrier.
Even though metal stanchions are widely used and accepted, the current steel design has several serious shortcomings. Considerable maintenance is required to prevent corrosion of the steel. In addition, the metal posts significantly contribute to a ship""s radar cross section, and are a source of electromagnetic interference when positioned near transmitting and receiving systems. The current steel stanchions can be permanently bent when bumped by aircraft carrier deck equipment.
Some stanchions used on aircraft carriers are designed to move, recessing into the deck when the ship""s aircraft elevator is up, and rising to provide a safety railing when the elevator is down. A permanently bent moving stanchion on a carrier is a critical problem, causing the ship""s elevator to become inoperable until the stanchion can be straightened or replaced. Even for non-moving stanchions, a bent stanchion represents a maintenance issue. An improved stanchion system, or more generically, any type of structural post that returns approximately to its undeformed position after a moderate impact, is highly desirable for use on aircraft carriers as well as general surface vessels. Any structural post or pole subjected to bending caused by operational overload or impact, such as sign posts, street lights poles, utility poles, flag poles, antennas and parking meters, would benefit from a capability to undergo large bending deformation followed by elastic recovery to the approximately original orientation.
The use of composite materials to replace steel in stanchions surrounding carrier elevators has been previously investigated. Stanchions made with traditional composite matrix materials such as polyester, vinyl ester and epoxy do reduce maintenance requirements associated with corrosion, and also reduce radar cross section and electrical interference issues. However, these materials result in stanchions that undergo a brittle failure, and therefore are too susceptible to catastrophic failure during frequent encounters with deck equipment. Matrix materials with higher strain to failure permit more damage tolerant stanchions to be constructed.
In previous experiments, a filament wound glass fiber reinforced polyurethane matrix composite stanchion was constructed by the Navy. The Navy""s experimental filament wound composite stanchion eliminates corrosion, reduces radar cross section, reduces the electrical interference issues associated with metal stanchions, and is suitably resistant to chemical and UV degradation. The experimental filament wound composite stanchion is less susceptible to damage that can incapacitate an aircraft carrier elevator system than prior metallic or composite stanchions. However, its elastic recovery after repeated impacts of the Navy experiment was less than ultimately desired. Cost of the filament wound system was also high. An improved stanchion design with much greater ability to elastically return from a bent position, and significantly reduced cost, is highly desirable.
A tubular stanchion or post providing a specified initial transverse load Capacity at a small deflection, coupled with the ability to recover its initial shape elastically when excessive accidental or impact loads are applied, is desirable in many military and commercial shipboard as well as other infrastructure applications. Additional stanchion attributes such as corrosion and UV exposure resistance as well as low electromagnetic cross-section would further improve the desirability of such stanchion configurations. The invention described herein provides all of these desirable functional characteristics through a composite material architecture. This architecture comprises a plurality of pre-pultruded unidirectional stiffeners oriented in the stanchion longitudinal direction and distributed uniformly about its circumference. The unidirectional pultruded stiffeners are sandwiched or encapsulated by inner and outer layers of a discontinuous and randomly oriented fibrous mat that provides both positioning of the pultruded stiffeners and a tailoring of the stanchion circumferential rigidity. Both the inner and outer reinforcing fibrous mat layers and the interstitial space between the unidirectional pultruded stiffeners are substantially filled with a highly resilient resin matrix. The principal result of this unique construction is that a tailored high longitudinal bending rigidity is coupled with a low circumferential rigidity.
The distinct elements of this basic construction may be varied to provide a pre-determined transverse load capacity at very small deflection, followed by a flattening of the critical cross section location and elastic bending to very large deflections when the preset threshold level is exceeded. The combination of highly resilient resin matrix and light random/discontinuous fibrous mat reinforcement causes the flattened/deformed stanchion to return elastically to its original configuration when the excessive load is removed. Suitable arrangement of the composite architecture details allows a stanchion so constructed to experience the low deflection threshold and large deflection response many times without exhibiting damage or loss of initial load capacity. The composite architecture included in this invention may be fabricated from a variety of types and proportions of constituent materials in numerous permutations suited to specific applications. Furthermore, the invention may be produced by any of several composite manufacturing processes, including pultrusion, resin infusion methods with hard or soft tooling, and manual lay-up procedures. For some applications it may be desirable to minimize or eliminate the fibrous mat reinforcement comprising the inner and outer surface layers of the composite stanchion, leaving the unidirectional pultruded stiffeners encapsulated by a neat or nearly neat resin matrix filling the spaces between the stiffener surfaces and the inner and outer diameters of the stanchion. Other aspects, features and advantages of the present invention are disclosed in the detailed description that follows.